Who knows how such data-driven giants can make such dumb, careless decisions, but they've made a right mess of the Blogger interface.
The old, simple method of pasting an embed code, checking just ONCE...EVER that interpret HTML is selected in OPTIONS has been mysteriously ditched - and you can't change the default to just display HTML.
So, I've figured out a way to do it.
Always do --- + return + ---, to paste into the blank line
type H + return a few times
Now, change the view from compose to HTML (the icon on the very left)
Look for the H's (you'll see br or p tags after them.
Paste your embed code in before the 1st H (as in my screenshot)
Switch back to compose mode + hit preview to check.
It should work...
Delete the H's!
Now say rude things about Google!
---
H
H
H
---
